2 Samuel 19:13

Say ye to Amasa. David was skillful in dealing with men in public office. Amasa had been the commander in chief for Absalom, and now that he was dead, Amasa was the leader who above anyone else could keep the spirit of the rebellion alive. Like Joab, Amasa was David’s nephew (1 Chron. 2:13-17), and David, by the dramatic move of making him commander in chief, sought to win Amasa’s loyalty. In turn Amasa would be expected to bring over to David what remained of the military organization of Absalom. Presumably David was restless under the overbearing influence of Joab and desired to be rid of him. It was through Joab’s influence that Absalom had been brought back to Jerusalem from his exile, and it was Joab who had slain Absalom in direct violation of David’s command. His recent cutting rebuke (vs. 5-7) was still ringing in David’s ears. David evidently felt that the time had come to replace Joab, and it was a shrewd political move to put Amasa in his place.